Abstract

An integrated approach to investigate the petroleum system of the southern Orange Basin, South Africa reveals that the principal controls on hydrocarbon generation and leakage are spatial and temporal variations in post-rift overburden deposition. The model predicts present day gas seepage at the sea floor and this is calibrated against observed seepage events.

D basin modelling of an east-west trending transect across the Orange Basin passive margin was undertaken accounting for variations in stratigraphy, structural deformation, source rock distribution and characteristics and heat flow variations associated with continental break-up. Petroleum generation and migration modelling was performed using compositional kinetic schemes, and migration and leakage routes calibrated to signs of current-day active leakage collected during seismic interpretation. The use of seismic indicators of hydrocarbon leakage as calibration data for constraining migration predictions is essential for the reconstruction of hydrocarbon generation and migration scenarios, as they provide the validation of the model with respect both to the physical properties and distribution of the sedimentary sequences as well as the dynamics of oil and gas migration. Modelling results indicate that the main period of hydrocarbon generation occurred in the Late Cretaceous, the time of maximum burial on the shelf. Following an erosion event at the end of the Cretaceous, deposition shifted over the shelf break and significant sediment volumes prograded into the deep basin. This led to a second pronounced maturation event focussed specifically under the prograding Tertiary wedges. The model predicts that the kitchen area is active today and is also the source of hydrocarbons that are observed to be seeping at the present day sea floor.

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